"The proposed system features onboard tanks containing a phase-changing polymer which would be cooled below its freezing point when the train was running on surface sections of the network. On underground sections heat transferred from the air-conditioning system would be used to melt the polymer again."

Thinking it'd be an amusing idea, and it actually existing; concrete with embedded hot-water pipes, for thousands of dollars. "Imagine, no shoveling, no loud snow plow, no recruiting neighbor kids to work on a cold day."